A mob turncoat yesterday fingered a Gambino hit man as the knife-wielding thug who fatally stabbed another mobster.

Former Bonanno associate Vincent Rossetti, 44, said he and a group of friends saw victim Salvatore Puma “getting his ass reamed” by accused killer Charles Carneglia on a Queens street corner in July 1983 and were too scared to intervene.

Then, Rossetti said, “He . . . was holding his chest. He . . . just collapsed . . . and he had a blood patch on his shirt.”